When did Mormon Jr. write The Words of Mormon?

Many hundred years after the birth of Jesus

It is many hundred years after the coming of Christ that I deliver these records into the hands of my son [Moroni]. The Words of Mormon 2

384 years after the birth of Jesus

When three hundred and eighty and four years had passed away [since the birth of Jesus] ... I made this record out of the plates of Nephi, and hid up in the hill Cumorah all the records which had been entrusted to me by the hand of the Lord, save it were these few plates which I gave unto my son Moroni. Mormon 6:5-6